Functional differentiation among 12 dipterocarp species under contrasting water availabilities in Northeast Thailand

Abstract

Species composition varies greatly dependent on water availability gradients. In Northeast Thailand, dry deciduous forests (DDF) and dry evergreen forests (DEF) show contrasting species composition due to differences in soil structure and moisture. Although plant traits (physiological and morphological characteristics) are known to be involved in species distributions, which traits underpin these distinct distributions (either dry DDF or less-dry DEF) remain unclear. Here, we examined the differentiation of 21 leaf and stem traits between DDF and DEF using 12 dipterocarp species. We found that DDF species showed higher water use efficiency and higher water storage capacity in the lamina and petiole, higher leaf nitrogen content, higher stomatal density, larger leaves, thicker mesophyll layers, and a higher rate of water loss under severe dehydration than DEF species. Leaf osmotic potential at full turgor, wood density, and wood water content were not significantly different between DDF and DEF. We also observed a negative relationship between the potential photosynthetic capacity and the water loss rate during severe dehydration across species. Our results suggest that the differences in leaf traits related to photosynthesis and dehydration avoidance among the tree species produce niche differences along the soil water availability in tropical dry forests.